1. Field of the Invention
The present invention relates to an information processing device that stores and/or reads necessary information by a contactless means such as wireless communication. In particular, the invention relates to a reader/writer device (also called an interrogator or a controller) that reads/writes information from/to an IC chip (also called an ID chip, an IC tag, an ID tag, an RF tag, a wireless tag, an electronic tag, or a transponder) for RFID (Radio Frequency Identification).
2. Description of the Related Art
With development of computer technologies and improvement of image recognition technologies, information identification utilizing media such as bar codes has spread widely and been used for identification of product data and the like. It is expected that the amount of information to be identified will further increase in the future. On the other hand, information identification utilizing bar codes is disadvantageous in that a bar code reader is required to be in contact with bar codes, and the amount of data stored in bar codes cannot be increased so much. Therefore, contactless information identification and increase in the storage capacity of media are required.
In view of the foregoing requirements, a contactless IC chip for RFID (hereinafter referred to as an IC chip) and a reader/writer device (hereinafter referred to as a reader/writer) have been developed in recent years. The IC chip has a memory circuit to store necessary information, and the information inside is read by a reader/writer using a contactless means, generally a wireless means. It is expected that practical application of an information processing device for reading information stored in such an IC chip allows commercial distribution and the like to be simplified and reduced in cost while ensuring high security.
An RFID system using an IC chip is briefly described with reference to FIG. 13. FIG. 13 is a block diagram showing as a conventional technology a reader/writer disclosed in Patent Document 1. FIG. 13 is configured by an IC chip 1301, a reader/writer 1302, and a high-level device 1303. The high-level device 1303 here exchanges data processing instructions and data processing results with the IC chip 1301 via the reader/writer 1302, thereby controlling identification of the individual information.
The reader/writer 1302 is configured by a receiving portion 1304, a transmitting portion 1305, a control portion 1306, an interface portion 1307, antennas 1308, and resonant capacitors 1309. The high-level device 1303 controls the control portion 1306 via the interface portion 1307 so that the control portion 1306 controls the receiving portion 1304 and the transmitting portion 1305 with respect to data processing instructions and data processing results. The transmitting portion 1305 modulates data processing instructions that are to be transmitted to the IC chip 1301, and outputs them as electromagnetic waves from the antenna 1308. The receiving portion 1304 demodulates electromagnetic waves received by the antenna 1308, and outputs them as data processing results to the control portion 1306.
In receiving data, the antennas 1308 and the resonant capacitors 1309 (hereinafter referred to as an antenna circuit 1310), which are connected to the receiving portion 1304 and the transmitting portion 1305 and configure an LC parallel resonant circuit, receive, as an electrical signal, electromotive force that is induced in the antenna circuit 1310 by electromagnetic waves outputted from the IC chip 1301. Meanwhile, in transmitting data, induced current is supplied to the antenna circuit 1310, and the antennas 1308 transmit electromagnetic waves to the IC chip 1301.
Although details are omitted, the IC chip 1301 also has an LC parallel resonant circuit configured by an antenna and a resonant capacitor, and electromagnetic waves are received from and transmitted to the antennas 1308 in the reader/writer 1302.
The aforementioned antenna circuit 1310 has a unique resonant frequency f0 that is determined by the inductance of the antenna 1308 and the capacitance of the resonant capacitor 1309. When individual information is exchanged in an RFID system, the resonant frequency f0 of the antenna circuit 1310 in the reader/writer 1302 is required to be substantially equal to a transmitting frequency fc that is outputted from the transmitting portion 1305. The resonant frequency f0 of the antenna circuit 1310 here is set so as to satisfy the formula (1), wherein L is the inductance of the antenna 1308 and C is the capacitance of the resonant capacitor 1309.f0=1/{2π(LC)1/2}  (1)
When the resonant frequency f0 is equal to the transmitting frequency fc outputted from the transmitting portion 1305, data can be transmitted and received the most effectively. Therefore, in the antenna circuit 1310, the resonant frequency f0 is preset to be equal to the transmitting frequency fc.
It is to be noted that the frequency transmitted and received is 125 kHz, 13.56 MHz, 915 MHz, 2.45 GHz, or the like, each of which is standardized by ISO or the like. Modulation and demodulation systems in transmitting and receiving data are also standardized (for example, see Patent Document 2).    [Patent Document 1] Japanese Patent Laid-Open No. 2001-250096    [Patent Document 2] Japanese Patent Laid-Open No. 2001-250393
As set forth above, the resonant frequency f0 of an antenna circuit connected to a reader/writer is determined by the inductance of an antenna and the capacitance of a resonant capacitor. However, in an antenna circuit connected to a reader/writer, the inductance of an antenna is not so sensitive to temperature while the capacitance of a resonant capacitor is sensitive to temperature.
Since a capacitor used as a resonant capacitor is required to be reduced in size, a ferroelectric-based capacitor with large capacitance is used. A ferroelectric typified by barium titanate, which has a high dielectric constant of 1000 or more, is widely used for electronic apparatuses and highly versatile. In the ferroelectric-based capacitor, however, electrostatic capacitance has relatively high temperature dependence and electrostatic capacitance and dielectric loss change significantly.
As a result, with changes in ambient temperature, the capacitance of a resonant capacitor in an antenna unit changes and the resonant frequency f0 of an antenna circuit changes as seen in the formula (1), which causes a problem in that a received voltage level is lowered in an IC chip that receives data.
A change in the transmitting frequency fc inputted to an antenna circuit, relative to temperature, can be suppressed to several ppm by using a crystal oscillator or the like in a transmitting portion. However, in an antenna circuit having a ferroelectric-based capacitor, the electrostatic capacitance has relatively high temperature dependence as described above, and the resonant capacitance may change by several percent in some cases. Accordingly, a problem occurs in that the resonant frequency f0 changes each time ambient temperature changes and a signal transmitted from an antenna circuit is attenuated.